Cooling it back down — answer key

1. SQ9 · Practice

Model answer. The flat part is at 53 °C, the freezing point. From W to X, T is a liquid. From X to Y it is freezing, so liquid and solid T are both present; the temperature stays constant because energy released as forces of attraction pull the particles into fixed positions replaces the energy lost to the room. At Z, T is a solid.

A full answer includes

• Temperature of the flat part: 53 °C (the freezing point).
• From W to X: liquid.
• From X to Y: T is freezing; liquid and solid are both present.
• The temperature stays constant because energy released as particles are pulled into fixed positions replaces the energy lost to the room.
• At Z: solid.

Also accept

• 53 °C; liquid; freezing, liquid and solid; energy released as particles are pulled together balances the loss; solid

Watch for

• “Between X and Y nothing is happening, because the temperature doesn't change.” — T is still losing energy to the room. Energy released as particles are pulled into fixed positions replaces it, so the temperature stays constant while T freezes.

2. SQ10 · Practice

Options as numbered on the pupil sheet: 1 — Solid; 2 — Liquid; 3 — Gas.

Model answer. Oxygen is a gas: 25 °C is above its boiling point of −183 °C. Bromine is a liquid: 25 °C is between −7 °C and 59 °C. Iron is a solid: 25 °C is below its melting point of 1538 °C.

A full answer includes

• Oxygen: gas, because 25 °C is above its boiling point.
• Bromine: liquid, because 25 °C is between its melting and boiling points.
• Iron: solid, because 25 °C is below its melting point.

Also accept

• Oxygen gas, bromine liquid, iron solid.

Watch for

• “Bromine is a gas because its boiling point is low.” — 59 °C is low for a boiling point, but it is still above 25 °C. Bromine has not reached its boiling point, so it is a liquid.

3. SQ14 · Practice

Model answer. 1, 3 and 5. In each one water vapour, a gas, loses energy at a cold surface and turns into liquid water. A puddle drying is evaporation; ice forming on a pond is freezing.

A full answer includes

• I chose 1, 3 and 5.
• I said each one is a gas (water vapour) turning into a liquid as it loses energy at a cold surface or in cool air.

Also accept

• 1, 3 and 5: water vapour losing energy and becoming liquid.

Watch for

• “Ice forming on a pond is condensing, because it happens when things get cold.” — Cooling causes both condensing and freezing. Condensing starts with a gas; ice forms from liquid water, which is freezing.

4. SQ11 · Practice

Model answer. (a) Condensing. (b) The cold glass takes energy from the steam. Its molecules slow down, and forces of attraction pull them close together, so they form liquid water. (c) Pure steam condenses at 100 °C, the boiling point of water; the temperature stays at 100 °C while it condenses.

A full answer includes

• (a) Condensing: a gas turning into a liquid.
• (b) At the cold glass the molecules lose energy and slow down, so forces of attraction pull them close together into a liquid.
• (c) 100 °C, the boiling point of water.

Also accept

• Condensing; the molecules lose energy at the cold glass, slow down and are pulled together by forces of attraction; 100 °C, the boiling point.

Watch for

• “The cold water around the tube leaks in and drips out.” — The cold water stays outside the glass. The drops come from the steam inside the tube, which condenses as it loses energy.

5. SQ12 · Practice

Model answer. Liquid. 37 °C is above gallium's melting point of 30 °C but far below its boiling point of 2400 °C, so it melts in the hand.

A full answer includes

• I chose liquid.
• I said 37 °C is above the melting point but below the boiling point.

Also accept

• Liquid, because 37 °C is between 30 °C and 2400 °C.

Watch for

• “Metals are always solid.” — The state depends on the temperature compared with the melting and boiling points. Gallium's melting point is below body temperature.

6. ST3 · Apply

Model answer. (a) 58 °C: the temperature stays at 58 °C from 4 to 7 minutes while V freezes. (b) At 1 minute it is liquid; at 5 minutes liquid and solid V are both present; at 10 minutes it is solid. (c) Solid: 35 °C is below its melting point, which is the same as its freezing point, 58 °C. (d) Ammonia condenses at −33 °C, its boiling point; the temperature stays at −33 °C while it condenses, because energy is released as the particles are pulled together.

A full answer includes

• (a) Freezing point 58 °C: the temperature stays constant from 4 to 7 minutes.
• (b) 1 minute: liquid. 5 minutes: liquid and solid. 10 minutes: solid.
• (c) At 35 °C V is solid, because 35 °C is below its melting point of 58 °C.
• (d) It condenses at −33 °C, its boiling point, and the temperature stays at −33 °C until all of it has condensed.

Also accept

• 58 °C (constant 4–7 min); liquid, liquid + solid, solid; solid at 35 °C because it is below the melting point; ammonia condenses at −33 °C and the temperature stays constant while it does.

Watch for

• “V freezes at 41 °C, the lowest temperature in the table.” — The last reading is just where the pupil stopped. Freezing happens where the temperature stays constant.
• “A gas condenses at its melting point.” — Condensing is boiling in reverse, so a pure gas condenses at its boiling point.